1. Field of the Invention
The present invention relates to a DC-DC converter having a large step-down ratio, and for example, to a DC-DC converter for changing a commercially available power supply voltage into a direct current and then converting the direct current into a low voltage.
2. Description of the Related Art
As is well known, DC-DC converters (direct current to direct current converter circuits) have been conventionally used in order to obtaining a direct current voltage by decreasing a commercially available power supply voltage. As the above DC-DC converters, buck type, boost type, and buck-boost type DC-DC converters are known as those using a transformer and a switching element, for example. In this case, by turning ON/OFF the switching element, an energizing current is supplied to a primary winding of the transformer, and a voltage generated at a secondary winding of the transformer is rectified so as to obtain a direct current output voltage.
In the meantime, in the case of a buck type converter including a transformer, an output voltage can be controlled by controlling an ON/OFF period of the switching element. Basically, assuming that an input voltage is Vin, an output voltage is Vout, a duty of the switching element is D, and a ratio (N1/N2) between the number N1 of primary windings of the transformer and the number N2 of secondary windings is “n”, the output voltage Vout is represented by:Vout=D·Vin/n  (1)Note that the duty D is indicated by D=Ton/Ts when it is assumed that a switching cycle of the switching element is Ts and an ON period of the switching element is Ton.
Therefore, there is a need for a DC-DC converter having a large step-down ratio to increase the winding number ratio “n” of the transformer. However, when the number of windings is large, a leakage inductance increases, and when a high frequency is produced, a frequency characteristic has been degraded. In addition, the winding number ratio “n” is high, and thus, there has been inconvenience that downsizing is difficult.
For this reason, even when a commercially available power supply voltage can only be stepped down to about 5 volts at most. In the case of obtaining a further lower voltage (for example, 2 to 3 volts), there has been employed a system of temporarily stepping down a voltage to an intermediate voltage, and then stepping up the stepped-down voltage again. In this case, the DC-DC converter is inserted in two stages, and there has been a problem with lowered efficiency and an increased number of parts.
The specification of U.S. Pat. No. 6,728,118 describes an exemplary step-down type DC-DC converter, in which an output voltage is specified in accordance with formula (1) above, and thus, stepping-down operation has been limited.
That is, in the conventional DC-DC converter, it is necessary to increase a winding number ratio of a transformer if an attempt is made to obtain an output voltage converted from a commercially available power supply into a low voltage. Thus, there has been inconvenience that a frequency characteristic is degraded and downsizing is difficult, and also stepping-down operation has been limited.